Current logging practices are undergoing significant changes due to the increased opposition to clear cutting as a means for harvesting timber. In traditional clear cutting, all trees are harvested, and in order to remove the fallen trees, conventional yarding equipment has been used to retrieve the logs from between stumps to a location where they are loaded onto trailers for subsequent transport away from the logging area. One way to harvest timber without clear cutting of this nature is to remove selected trees from a given area while leaving many existing trees standing. This selective harvesting of timber is often referred to as thinning.
In order to meet demands of better thinning and selective harvesting of timber, logging equipment has been developed that allows fallen trees to be removed by a more circuitous route through the trees that remain standing than by using the straight, clear-cut path retrieval of conventional line systems. Such logging equipment can transport harvested trees along a meandering path to a location where they are loaded on trailers for subsequent transport from the logging area. One such system for thinning trees is known as a mono-cable system. The mono-cable system relies on a continuous loop of cable that is strung through a logging area along the path that the harvested timber is conveyed. The cable is generally fixed in length and driven by a mechanical means such as a hydraulic motor. Conventional choking equipment, including chokers, hooks, and stoppers, are used in conjunction with the cable to secure felled trees to the cable. Logging blocks having radially extending teeth are secured to standing trees or stumps, and are used to support and guide the cable along the path, as well as to change its direction through the logging area.
In one existing mono-cable system, the cable is provided with a plurality of annular sleeves secured along the length of the cable at intermittent locations. These sleeves act as the stoppers for the hooks that hang freely on the cable from one end and have at their opposite end a choker for securing a log or logs. In operation, the choker can be slid along the cable to a felled log and, if necessary, the cable itself can be advanced to assist in positioning the choker with respect to a log. Once the log is secured, the mono-cable system is advanced, causing the cable to slide through the hook until a stopper is reached, whereupon the stopper engages the hook and advances it along with the cable. The cable, stopper, hook, choker and secured log or logs are then transported along the meandering mono-cable system path--along the path determined by the location of the blocks--to the point where they are conveniently loaded onto trailers.
Conventional logging blocks used in such mono-cable systems include a sheave or pulley that has a groove or channel in which the mono-cable is seated during normal operation. The groove is defined between two rails on opposite sides of the sheave. One rail is substantially smooth and has an outer diameter that is equal to or greater than the diameter of the groove in the sheave. The opposite rail may also be substantially smooth or, as is more common, comprise a plurality of radially extending teeth. In the conventional blocks, these teeth are machined into the sheave rail or are rigidly affixed thereto. The sheave and corresponding rails are rotatably mounted to a frame which serves to carry the combination and secure it to a tree or stump via a harness such as a cable or belt. The harness is looped around the tree or stump and through a notch or hole cut into the frame, thereby anchoring the block to the tree or stump.
When tension on the mono-cable is increased by activating the winch, the blocks which have been secured to a tree or stump are extended horizontally relative to their anchor by the force of the taut cable in the direction away from the tree or stump. In this position, the side of each block with the radial extending teeth is below the smooth rail and essentially parallel with the ground. During normal operation, the cable rides in the groove of the sheave. As a portion of the mono-cable carrying the hook enters the block and begins to pass along the groove of the sheave, the vertically oriented hook either seats itself between the protruding teeth or contacts a tooth along its sides or at its end and the protruding teeth advance the cable and associated hook and sleeve along the sheave.
Because of the high tension on the mono-cable, the potential for injury to hookers or other logging personnel in the area is great if the cable jumps the block. Additionally, the down time involved in restringing the mono-cable onto a block adds to the overall cost of the logging operation. The potential for the hook assembly to jump off the logging block relates to a number of factors, not the least of which is the ability of the sheave to twist to maintain proper alignment with the incoming weighted cable. More specifically, as a portion of the mono-cable carrying the hook approaches the horizontally extended sheave, the weight of the associated secured log pulls downward against the cable and sheave on the side of the sheave closest to the approaching hook. This downward force tends to twist the sheave on its anchor away from horizontal and towards alignment with the approaching weighted cable. This aligns the protruding teeth of the sheave with the weighted cable and hook so that the hook correctly seats itself between the teeth. Twisting of the sheave is therefore necessary to prevent or reduce the likelihood that the cable will jump off the logging block.
The ability of the sheave to twist to accommodate the incoming weighted cable and hook depends on the amount of cable contact with the sheave. Very little cable contact reduces the twisting force on the sheave as the weighted cable and hook approaches. The cable itself rather than the sheave is likely to be forced downward and possibly off the block. Conversely, greater cable contact with the sheave forces the sheave itself to twist.
Perhaps the most significant factor affecting cable contact with the sheave is the angle between adjacent logging blocks. The smaller the angle between adjacent logging blocks, the greater the cable contact with the surface of the sheave. Traditional logging blocks in existing mono-cable systems are anchored on nearby trees along the desired mono-cable path in roughly the same plane such that the angle at any given logging block between adjacent logging blocks to either side is approximately 90.degree. to 125.degree.. This allows a meandering mono-cable path that is predominately jagged in appearance. By aligning the harness looped through the notch or hole cut into the frame of the logging block with the center of the groove of the sheave, maximum force on the block extending it horizontally away from the anchor--and therefore maximum contact between the cable and the surface of the sheave--is ensured at any given moment during mono-cable system operation within this range.
As the angle between adjacent logging blocks is increased beyond approximately 120.degree., however, such as in an attempt to obtain a straighter mono-cable path, less cable is in contact with the sheave. Equivalent cable force on the logging block at the higher angle therefore increases the pull of the cable away from the surface of the sheave. This decreases the cable force extending the block horizontally away from the anchor. Reduced cable force maintaining the mono-cable on the sheave decreases the ability of the sheave to twist to align itself with the approaching hook and log. This, in turn, increases the likelihood that the cable will jump the block.
The distance between adjacent logging blocks also affects the ability of the sheave to twist. The shorter the distance between the current logging block (through which the weighted cable is passing) and the subsequent logging block (the next logging block along the mono-cable path), the less flexibility the current logging block sheave has to twist. The closer the two sheaves the more tension that exists along the cable between the sheaves, thus constraining the twisting of the current sheave. The greater the distance between these sheaves, however, the less tension along the cable, and the greater the twisting flexibility of the current sheave.
Given that the topography of each logging area varies tremendously, the distance between adjacent logging blocks in a mono-cable system cannot always be ideally set to achieve the necessary sheave twist. The spacing between trees, for example, may force placement of logging blocks closer to each other than desired along a given mono-cable path. To compensate for the variance in distance between logging blocks mandated by the logging area, the relative plane in which adjacent logging blocks sit may be adjusted. While anchoring a subsequent logging block relatively close to a current logging block in the same relative plane reduces possible sheave twisting in response to approaching hooks with associated secured logs, anchoring the subsequent block in a higher plane relative to the current block prestresses the block to twist towards the approaching hook. Adjusting the relative plane in which adjacent logging blocks sit thus compensates for variances in distance between blocks.
In sum, the potential for the hook assembly to jump off the logging block is related to (1) the angle between adjacent logging blocks, (2) the distance between adjacent logging blocks, and (3) the relative plane in which adjacent logging blocks sit. Each of these factors affects the forces between the cable and the surface of the sheave and, thus, the proper functioning of the mono-cable system.
Accordingly, to avoid risking injury and additional costs associated with the cable jumping the block in mono-cable system operation, the placement of traditional logging blocks along a mono-cable path has heretofore been extremely limited by angular range, plane, and distance between adjacent logging blocks. This limitation restricts logging personnel in their choice of mono-cable paths, forcing them to adopt paths that are either not as efficient in maximizing thinning efforts for any given tree stand, or not possible given the location of existing stumps and the topography of the logging area.
Although mono-cable systems and conventional blocks are in use, there is a need for an improved logging block that provides increased flexibility in the placement of logging blocks between mounting trees forming a mono-cable path while reducing the likelihood that the mono-cable will jump the block. By so doing, the improved logging block would increase the usefulness and efficiency of the mono-cable logging system while reducing the risk of injury to logging personnel and associated economic loss.